When municipal wastewater treatment plants clean up sewage, they never fully remove some types of contaminants. The plants don't track or treat some chemicals, such as pharmaceuticals, so these compounds remain in the water, or effluent, that plants release into the environment. Now Canadian researchers report that effluent can cause metabolic stress in rainbow trout, which could harm the long-term health of fish populations (Environ. Sci. Technol., DOI: 10.1021/es103122g).

Sewage treatment primarily reduces or removes trash, debris, organic matter, and disease-causing organisms from wastewater. Scientists worry about the impacts of the remaining compounds on aquatic organisms. Previous studies have investigated how individual chemicals or classes of these chemicals affect animals, particularly through their endocrine systems. Jennifer Ings, a graduate student at the University of Waterloo, and her colleagues wanted to understand how animals responded to a realistic mix of the substances that linger in treated municipal wastewater.

The researchers placed cages containing eight to16 juvenile trout (Oncorhynchus mykiss) at five locations in the Speed River in Guelph, Ontario. Three of the sites were downstream of a municipal wastewater treatment plant. The water at these spots consisted of 100%, 50%, and 10% effluent. The final two sites were upstream of the plant and served as controls. After 14 days, Ings and her team collected the cages and examined the fish in their lab, performing analyses of hormone, protein, and gene expression levels.

Trout from the downstream sites, particularly the 50% and 100% effluent sites, showed increased cortisol and plasma glucose levels, both indicators of metabolic stress. The team also detected significant changes in the expression levels of 27 genes, several of which play roles in metabolism and stress response. The researchers concluded that fish exposed to the wastewater ramped up their energy use. If the exposure became chronic, says co-author Matt Vijayan, the extra energy expenditure could disrupt the animals' resilience, reproduction, and immune response over the long term.

Fish toxicologist Anne McElroy of the State University of New York, Stony Brook, says the study is important because it examines fish in actual wastewater effluent and takes a "holistic approach" by connecting changes in gene expression to changes in proteins and hormones. These observed gene and biochemical changes could lead to future health problems, she says.

The study reveals that current best practices for cleaning municipal wastewater fail to prevent possible ecological harm, says Vijayan. He points out that even after these thorough clean up techniques, this plant's effluent still had a biological effect on the fish.